1. Field of the Invention
The present invention relates to a series of carbazole derivatives which can be used in organic electroluminescent devices as triplet host materials of the emissive layers.
2. Description of the Related Arts
With the development of the multimedia technology and the coming of the information society, the requirement to the flat panel display devices is higher and higher. The three kinds of new display technology, i.e. Plasma Display Panel (PDP), Field-Emission Display (FED) and Organic Light Emitting Devices (OLED), can surmount the shortcomings of the cathode ray tubes and liquid crystal display to a certain extent. The OLED has the properties of self-emission, low voltage DC driving, all-solid materials, wide visual angle and full color display, etc. Comparing to the LCD, the OLED doesn't need background illumination, has larger visual angle, costs less energy and its response speed is 1000 times more than that of the LCD while its cost is lower than that of LCD with the same resolution. So, OLED will have potential applications in many fields. In 1987, C. W. Tang et al of Kodak (C. W. Tang, S. A. Vanslyke, Appl. Phys. Lett., 1987, 51, 913) used 8-hydroxyquinoline aluminium (Alq3) as the emitting layer and got an OLED having driving voltage of lower than 10V, luminescence of higher than 1000 cd/m2 and the lifetime of more than 100 hours. OLED exhibits its potential practibility.
OLED works in accordance with the following mechanism: when a bias is applied across an anode and a cathode, electrons and holes are respectively injected from the anode and the cathode into the organic materials and then are transferred to the interface to combine and to form excitons which can emit light. OLED typically comprises a low-work-function cathode, a high-work-function anode and one or more layers of organic emissive material located between the anode and the cathode. The organic emissive layer may comprise at least one of electron-transporting layers, hole-transporting layers and emissive layers.
Although the research of OLED is developing rapidly, there are still some problems to be solved urgently and the most important one is that the quantum efficiency and stability of the devices can't reach application level. The EL quantum efficiency of the devices reflects the integration of all the factors and is an important index for evaluating quality of the devices. Generally, EL quantum efficiency of a device is represented by external quantum efficiency which is the fraction of the numbers of photons getting out of the devices and the numbers of carriers injected to the devices.
The OLED external quantum efficiency meets theoretically the following equation:ηqe=χΦFηrηe
wherein ηqe is the external quantum efficiency, ΦF is the intrinsic photoluminescent quantum efficiency of the emissive materials (including both fluorescence and phosphorescence) and (ΦF≦1, ηr is the probability of the formation of excitons and ηr≦1, ηe is the light out-coupling efficiency and ηe≦1, χ is the fraction of the formed total excitons which result in different radiative transitions and is about ¼ in relation to the singlet exitons while ¾ in relation to the triplet exitons. According to the above equation, the approaches of improving the external quantum efficiency of the devices may include: 1) using the emissive materials with high ΦF, 2) improving the probability of the formation of excitons, 3) improving the fraction of the photons getting out of the devices, and 4) improving the fraction of the available excitons.
Practically, the light out-coupling efficiency is up to 20%. The fluorescent molecular dyes can only use the singlet exitons and the external quantum efficiency of OLEDs is up to 5%. The triplet materials can use all of the formed exitons (both singlet and triplet exitons) and the external quantum efficiency of OLEDs can reach to 20%. So, use of the triplet emissive materials can improve the external quantum efficiency of OLEDs notably.
However, the triplet emissive materials have the triplet-triplet quenching when the concentration of triplet exitons is relatively high. So the triplet emissive materials should be doped into the host emissive materials instead of using itself alone as the emissive layers. In the energy transfer from the host to the guest materials, it requests the host materials have a higher energy level, so the host materials doped by the triplet emissive materials should have a higher triplet energy level.
Princeton University and University of Southern California worked together and in the U.S. Pat. No. 6,303,238 which was filed in December 1997 and published in October 2001 they first reported the triplet materials were used as the dopant to fabricate high efficiency OLEDs. This patent and other succeeding literatures written by the Forrest group from Princeton University (for example, M. A. Baldo, D. F. O'Brien, Y. You et al. Nature, 1998, 395, 151) published the research of using the triplet materials 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (II) (PtOEP) as the guest emissive materials doped in Alq3. Owing to the limit of spin-forbidden, the usage of excitons in fluorescent small molecule OLEDs is very low (the highest value is 25% in theory) and thus this limits the external quantum efficiency of fluorescent OLEDs (lower than 5%). On the other hand, if the triplet materials are used, the usage of excitons can reach 100% in theory and will improve the efficiency of the devices notably. Generally, the emission of the triplet materials at room temperature is quite low. However, PtOEP can emit phosphorescence at room temperature because the heavy metal Pt is introduced to the porphyrin rings and increases the rate of the intersystem crossing and the phosphorescence can be emitted at room temperature. The configuration of the device is represented as:
ITO/CuPc(6 nm)/NPB(35 nm)/Alq3:PtOEP(6% wt,40 nm)/Alq3(10 nm)/Mg:Ag(25:1, 100 nm)/Ag(50 nm)
Wherein CuPc is copper phthalocyanine, NPB is N,N′-bis(1-naphthyl)-N,N′-bis(phenyl)-benzidine, the device has an emissive peak at 650 nm at different current densities and has no peak at 580 nm while the PtOEP has a fluorescence peak at 580 nm and has a phosphorescence peak at 650 nm. This suggests that the emission of the device is originated from the phosphorescence of PtOEP. The internal quantum efficiency of the device reached as high as 23% at low luminescence and the external quantum efficiency was nearly 5%. However, the external quantum efficiency reduced to 1.3% at high luminescence (100 cd/m2) and exhibited the emissive peak of Alq3. This indicates that the energy transfer between the Alq3 and PtOEP is not sufficient and the efficiency of the energy transfer is quite low.
In order to improve further the efficiency of the PtOEP phosphorescence devices, the Forrest group (D. F. O'Brien, M. A. Baldo, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett. 1999, 74, 442) substituted 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP, represented by formula I) for Alq3 as the host material of the triplet dopant PtOEP, and introduced 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as the hole blocking layer after the emissive layer. The configuration of the device is represented as:
ITO/NPB(45 nm)/CBP:PtOEP(6%,40 nm)/BCP(8 nm)/Alq3(25 nm)/Mg:Ag
It was found that the external quantum efficiency of the device was as high as 5.7%, and higher than the theoretical maximum 5%, which proved that the triplet emissive materials could gain higher efficiency. Comparing to Alq3, CBP has higher triplet energy (CBP, 2.56 eV; Alq3, 2.0 eV), the ability of ambipolar transfer. So it is more likely to reduce the back energy transfer from the dopant to the host materials and to increase the energy transfer from the triplet exitons of the host to the triplet emissive dopant materials. From then, CBP has been widely used as the host materials of many triplet emissive dopants and achieved high EL quantum efficiency for red, green, blue and white electrophosphorescence.

As the development of the research on phosphorescent electroluminescent devices, Princeton University and University of Southern California worked together to develop more and more triplet dopant with good performances. But the research on the host materials is relatively insufficient. The previous host materials used for fluorescent materials are not always suitable for the triplet emissive materials. So the research on the triplet emissive host materials is in great need.
The host materials for red or green triplet emissive materials are quite enough and CBP is the most-widely used materials. Some polymers such as polyvinylcarbazole (PVK) are also used as the host materials for the triplet dopant materials. But the host materials for blue triplet emissive dopants are rather few. For example, the commercially available blue triplet dopant material, iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate (FIrpic, represented by the following formula II), was the first reported blue triplet emissive material and the emissive peak is at 475 nm at room temperature and the triplet energy level is 2.65 eV. However, the triplet energy level of the classic host material CBP is 2.56 eV. The Forrest group (Chihaya Adachi, Raymond C Kwong, et. al. Appl. Phys. Lett. 2001, 79,2082) used CBP as the host emissive materials of FIrpic and fabricated blue electroluminescent devices which have the external quantum efficiency of 5.7%, the maximum operation current density of 100 mA/cm2 and the luminescence of only 6500 cd/m2. The triplet energy level of CBP is lower than that of Firpic and the energy transfer from CBP to Firpic is an endothermic process. The efficiency of the endothermic transfer is very low and the energy transfer from the Firpic to CBP limits the efficiency and lifetime of the devices. In order to improve the efficiency and lifetime of the blue electroluminescent phosphorescent devices, the host materials with higher energy level are required.

The blue organic electroluminescent devices are absolutely necessary for the full-color display. So the blue electroluminescent devices with high efficiency and long lifetime are quite important. Both the theoretical research and the experimental records proved that the triplet emissive materials are helpful to improve the EL efficiency of the OLEDs. Developing host emissive materials with high energy level for blue triplet emissive materials is a hot and challenging subject matter.
Recently, the Forrest group (R. J. Holmes, S. R. Forrest, et al. Appl. Phys. Lett. 2003, 82, 2422) reported a new type of carbazole derivative, N,N-′-dicarbazolyl-3,5-benzene (mCP), of which the triplet energy is 2.90 eV. After using mCP as the host emissive materials and Firpic as the dopant, the maximum external quantum efficiency of the device was 7.5% and it was 30% higher than the device used CBP as the host emissive materials. That was because the energy transfer from mCP to Firpic is an exothermic process and the energy transfer was more efficient. Later, Tokito et al (Shizuo Tokito, Toshiki Lijima, et al. Appl. Phys. Lett. 2003, 83, 569) reported a type of CBP substituted by two methyls, 4,4′-bis(9-carbazolyl)-2,2′-dimethyl-biphenyl (CDBP), with the triplet energy level of 3.0 eV, and the external quantum efficiency of the device doped with Firpic was as high as 10.4%.
Both mCP and CDBP have higher triplet energy than the blue triplet dopant FIrpic (2.65 eV). As a result, the devices using mCP and CDBP as host materials and using Firpic as dopant gained high quantum efficiency. But the thermal stability and the film-forming capability of the mCP and CDBP are quite low. The glass transition temperature of mCP is only 65° C. and the organic emissive layer is easy to crystallize. Furthermore, the maximum operation current density is only 100 mA/cm2 and the higher driving current will damage the devices. So, in the search of the host material with high energy, the thermal stability is also considered. In the Chinese patent application publication No. CN1365381A, a series of carbazole derivatives were designed, which have the glass transition temperature of above 110° C. and the triplet energy of above 21000 m−1 (2.56 eV, according to the wavelength at 488 nm). These carbazole derivatives were used as dopants in the OLEDs with the triplet emissive dyes. These carbazole derivatives comprise a triphenylamine group directly linked to the aromatic group as the center unit or carbazole oligomer. It was discovered from the preferred structures that along with the number of repeated aromatic groups increase, the conjugation of the molecule increase so that the triplet energy would reduce.